I investigated a reform based teachers' beliefs about the nature of mathematics, teaching mathematics, and learning mathematics, and the factors leading to their formation. I interviewed and observed a reform-based elementary mathematics teacher with 13 years' experience teaching first grade. She held a Platonist/problem solver view of mathematics, explainer view of teaching, and an active view of learning and was influenced by her husband, preschool son, past teachers, certification, professional development, and teaching experience. I found personal factors outside of education could significantly support beliefs in reform-based teaching and speculate that reflection on personal factors could initiate belief change. [Publication year printed on the pdf is incorrect, the correct publication year is 2017.]